Normally a steering system such as a power-assisted steering system has a track rod, drag link or the like, whose movement may be controlled hydraulically and which has a ball joint at each end. The ball joints are respectively connected to control or steering arms which are themselves connected to king pins mounted at the ends of the axial beam.
In an hydraulically-operated power assisted steering system using a track rod, for example, it is usual for an operating piston to be connected to the track rod. When the wheels are moved from full lock in one direction to full lock in the other direction the track rod becomes subjected to a twisting couple which tends to rotate the rod about its longitudinal axis, due to the inclination of the hydraulic cylinder in two planes relative to the track rod. This couple begins to act as soon as the wheels are moved from the straight-ahead position and the effect of this couple is to rotate the bodies of the ball joints at each end of the track rod around the spherical head of the ball pins about an axis along the length of the track rod, so that the necks of the ball pins strike against the edges of the sockets in the body of the track rod ends which receive the ball pins. Over extended periods under heavy duty service this may result in fatigue of the ball joints.